added abort to the SCTPTransport Stop

the association should send the abort message before closing the association as defined by the wbertc spec in step 6
https://www.w3.org/TR/webrtc/#dom-rtcpeerconnection-close

sctptransport.go

@@ -146,13 +146,14 @@ func (r *SCTPTransport) Start(SCTPCapabilities) error {
 	return nil
 }
 
-// Stop stops the SCTPTransport
+// Stop send the abort message and stops the SCTPTransport
 func (r *SCTPTransport) Stop() error {
 	r.lock.Lock()
 	defer r.lock.Unlock()
 	if r.sctpAssociation == nil {
 		return nil
 	}
+	r.sctpAssociation.Abort("")
 	err := r.sctpAssociation.Close()
 	if err != nil {
 		return err